1. Field of the Invention
This invention relates generally to marine seismic surveying, and, more particularly, to positioning a center of a seismic source used in marine seismic surveying.
2. Description of the Related Art
Seismic exploration is widely used to locate and/or survey subterranean geological formations for hydrocarbon deposits. Since many commercially valuable hydrocarbon deposits are located beneath bodies of water, various types of marine seismic surveys have been developed. In one type of marine seismic survey, the survey vessel tows a seismic source including a plurality of guns, such as airguns and the like, which are deployed along one or more strings or cables. As the source is towed over the survey area, the guns are fired to produce an acoustic signal, or “shot.” The shot is directed down through the water into the earth beneath, where it is reflected from the various subterranean geological formations.
The reflected signals are received by one or more seismic sensors. In various alternative embodiments, the seismic sensors may be deployed in a receiver array including one or more seismic streamer cables, at the sea bed, in a borehole, or in any other desirable location. The received signals are typically digitized and then transmitted to the survey vessel or other data collection location. The digitized signals are referred to as “traces” and are recorded, and at least partially processed, at the survey vessel. The ultimate aim of this process is to build up a representation of the subterranean geological formations beneath the array. Analysis of the representation may indicate probable locations of hydrocarbon deposits in the subterranean geological formations.
The representation of the earth strata in the survey area is formed by combining data collected along a plurality of sail lines. In one embodiment, the sail lines are approximately straight lines that cross a portion of the survey area. However, persons of ordinary skill in the art should appreciate that the sail lines are rarely, if ever, perfectly straight. For example, wind, water currents, waves, steering of the survey vessel, and the like may cause the sail line to be less than perfectly linear. Furthermore, the data may be collected along paths that are not intended to be linear. For example, multi-azimuth coverage of the survey area may be provided by surveys using non-linear paths having shapes including elliptical paths, circular paths, and figure-8 paths.
In one embodiment, a single survey vessel may tow a single receiver array along each of the sail lines. Alternatively, a plurality of survey vessels may tow a plurality of receiver arrays along a corresponding plurality of the sail lines. In various alternative embodiments, the data may be collected during a single survey conducted over a short period of time, e.g. approximately one day, or it may be collected in multiple surveys performed a different times. For example, inclement weather and/or high seas may force a survey to be suspended before resuming hours or days later. For another example, historical data from previous surveys performed months or years earlier may be combined with new data to extend the survey or to fill in deficiencies in coverage that may be introduced by currents, obstacles such as platforms, and the like. And for yet another example, data from repeat surveys may be used to analyze and monitor changes in productive oil and/or gas reservoirs.
The accuracy and/or resolution of the representation formed using the acquired data may be limited by uncertainties in the actual path of the seismic source and/or the receivers through the water. Accordingly, the survey vessel typically attempts to tow the seismic source so that a geometric center-of-source of the guns in the seismic source follows a desired sail line. However, the center-of-source rarely, if ever, precisely follows the desired sail line. For example, water currents, wind, waves, and the like may divert one or more guns in the seismic source from the desired path.
The present invention is directed to addressing one or more aspects of the problems described above.